1. Field of the Invention
The present invention relates to an exposure apparatus and method for aligning a wafer and mask and transferring a pattern on the mask onto a wafer.
2. Description of the Related Art
Conventionally, in an exposure apparatus for manufacturing semiconductor integrated circuits, an improvement in relative alignment precision between the mask and wafer is an important factor upon improving the performance. Taking a DRAM as a representative semiconductor integrated circuit as an example, total overlap precision around 1/3 to 1/4 of the minimum line width is required, and recently, an alignment precision of 20 nm or less is required due to highly integrated semiconductors. Of such precision values, an alignment precision required for the exposure apparatus is as strict as 10 nm to 15 nm.
In many exposure apparatuses, positional deviations of marks for alignment respectively formed on a mask and wafer, i.e., alignment marks, are optically detected, and the mask and wafer are aligned based on the detection value.
Methods of detecting the positional deviation include a method of projecting the alignment marks onto a CCD by optically enlarging them, and processing the projected images, a method of measuring the phase of light diffracted by linear diffraction gratings used as the alignment marks (Japanese Patent Laid-Open No. 62-261003), a method of detecting the positional deviation of light diffracted by zone plates (grating lenses), used as the alignment marks, on a predetermined plane (U.S. Pat. No. 4,037,969), and the like.
In any of these detection methods, alignment is done by driving the mask or wafer so that the positional deviation between the alignment marks on the mask and those on the wafer becomes equal to or smaller than a predetermined amount, and exposure is then made.
FIG. 16 shows the alignment method disclosed in Japanese Patent Laid-Open No. 62-261003. Linear gratings 135 serving as alignment marks are formed on a mask 141, and linear gratings 136 serving as alignment marks are formed on a wafer 142. These marks are irradiated with alignment beams 137 and 138 (frequencies f1 and f2) from two sides, and light beams 139 and 140 diffracted by the respective diffraction gratings are received. Then, the positional deviation between the marks 135 and 136 is measured based on the phase difference between the two diffracted light beams.
However, when exposure is made after the marks on the mask and wafer are aligned in this way, the alignment marks on the mask are transferred on or in the vicinity of those on the wafer, as shown in FIG. 17A. For this reason, the next layer cannot use the same wafer alignment marks, and alignment marks must be updated in turn, as shown in FIG. 17B.
Hence, mask marks and wafer marks must be formed at different positions in units of layers. The alignment marks of the wafer are normally located on a dicing margin called a scribe line, and other marks such as a CD evaluation pattern, overlay precision evaluation pattern, and the like are also located on the scribe line. For this reason, when the alignment marks require a larger area, other patterns cannot be formed.
In order to solve such a problem, the width of the scribe line must be increased. However, when the scribe line is made thicker, the number of shots on the wafer decreases, resulting in an increasing in the IC manufacturing cost. Since the number of layers is expected to grow in the manufacture of future devices, such a problem cannot be ignored.
When the alignment marks are updated, the exposure apparatus must have a stage system for driving an alignment detection system to trace the marks, resulting in high apparatus cost.
Furthermore, in a proximity exposure apparatus that uses, e.g., X-rays as an exposure light source, exposure light must be prevented from being intercepted by an alignment optical system upon exposure. For this purpose, the alignment system must be removed from the field angle upon exposure, or the alignment beams 137 and 138 are obliquely radiated to fall outside a plane determined by lines A1A1' and C1C1' perpendicular to the detection direction of the alignment marks, and light-receiving sensors are located on the side facing alignment beam projecting devices so as not to intercept the exposure light. However, such methods require complicated systems and high apparatus cost.